Diffusion process for the manufacture of a malleable iron-chromium alloy



DIFFUSION PkOCESS FOR THE MANUFACTURE O A MALLEABLE IRON-CHROMIUM ALLOY Filed July 25, 1960 Jan. 21, 1964 E ca. WEATHERLEY F 3,118,762

United States Patent 3,113,762 DlFFUSlG-N PRQCES FOR TIE MANUFACTURE Gi A MALLEABLE lRfiN-CHRGMIUM ALLOY Eric George Weatherley, 32 Niagara Ava, Northfield, Ealing, London W. 5, England Filed .luly 25, 1960, Ser. No. 45,169 Claims. (Cl. 75-1305) The invention relates to metals, particularly chromium.

According to the invention, there is provided malleable and/ or ductile chromium, for example, in sheet, strip or other form.

According to the invention moreover, a method for the production of a chromium-iron alloy comprises subjecting a sheet of mild steel or of wrought iron to a chromium diffusion process under condition such that not less than and preferably between 20% and 65 for example, between 50% and 65%, by weight of the iron in the sheet is replaced by chromium.

According to the invention furthermore, a method for the production of malleable and/or ductile chromium comprises subjecting a sheet of mild steel or of wrought iron to a chromium diffusion process under conditions such that not less than about 70%, preferably not less than 90%, of the iron in the sheet is replaced by chromium.

The sheet is preferably packed in a mixture of finely divided chromium metal and an inert diluent, preferably a sand of a high degree of purity, and maintained at a temperature, for example, 10301400 C., for a time sufiicient to permit the requisite amount of iron to be replaced by chromium.

The grain size of the chromium metal is preferably less than 60 3.8.5. The sand is preferably such that all of its particles are less than 32 B.S.S. in size and preferably not less than 70% of the sand is less than 72 13.8.5. The sand should not, however, be so fine as to prevent gaseous diffusion through its mass. The sand preferably contains not less than 99.3% silicon dioxide and not more than 0.05% of iron estimated as ferric oxide.

The diffusion process is preferably carried out in the presence of one or more additives effective to facilitate the diffusion of chromium into the mild steel or wrought iron sheet. Such additives comprise an ammonium halide, particularly ammonium chloride, a substance, such as ammonium fluoride which is effective to evolve hydrogen fluoride at the emperature at which the process is effected, and a compound which dissociates with the evolution of nitrous oxide at the temperature at which the process is carried out. ammonium nitrate. The chloride should be present in an amount of not less than 0.05% by weight of the sand/chromium mixture, the fluoride should constitute not less than 0.2% by weight of the mixture and the nitrate should constitute not less than 0.01% of the mixture. The preferred amounts of ammonium chloride, ammonium fluoride and ammonium nitrate are about 0.1%, 0.3% and 0.04% by weight respectively of the sand/ chromium mixture.

The purity of the mild steel employed has been found to be important. If the content of impurities is high, the replacement of the iron by chromium is at least hindered and the requisite degree of exchange of the two metals may even be prevented. The mild steel should therefore not contain too much phosphorus, sulphur, manganese, silicon or carbon. It is preferred that these elements should not be present in amounts greater than P 0.05%, S 0.05%, Mn 0.5%, Si 0.15% and C 0.12% in the ENZA mild steel. Such contents of these ingredients may, in fact be regarded as maxima for effective operation. ENZA grade of mild steel having carbon, silicon, manganese, sulphur and phosphorus contents considerably be- A suitable compound for this purpose is low the maximum amounts allowed in the 13.8. specification has been found to be the most suitable.

There is a limit to the thickness of the mild steel which can be used. For example, it has been found difiicult to replace more than 90% of the iron in a mild steel sheet of a thickness of 0.008 inch or thereabout. The thinner the sheet the greater the amount of iron which can be replaced by chromium.

The temperatures under which the process of the invention can be carried out may vary within a relatively wide range. The minimum temperature is probably about 1000l020 C. In one example, a sheet 0.005 inch thick of suitable ENZA mild steel was treated in accordance with the invention at a temperature of 1050 C. for 10 hours. The resulting malleable sheet contained 94% chromium. At 1200 C. the same result will be obtained in about 34% hours whilst at 1400 C. the same result is obtained in a very much shorter time.

The process is carried out in a container into which furnace or other deleterious gases cannot enter. The container is advantageously sealed with a substance which is liquid at the temperature at which the diffusion is carried out. The sealing composition is preferably saturated with chromium to prevent or minimise loss of chromium from the sand/chromium composition. The sealing composition may be a solid, liquid or gelatinous mass at atmospheric temperature, which forms a liquid glass or glass-like substance at the temperature of diffusion. The melting point of the sealing composition may readily be adjusted, for example, within the range 800- 1400 C., by varying the nature or proportion of its alkali content. It is preferred that the sealing composition should be liquid at a temperature 100 degrees C. or more below the temperature at which diffusion is to be effected.

A suitable sealing composition can be made by dissolving 100 parts of sodium tetrasilicate, 100 parts sodium dichromate and 10 parts sodium hydroxide in parts water, or by mixing together parts sand, 40 parts anhydrous sodium carbonate and 21 parts finely divided chromium metal, all parts being by weight.

The container should be cooled under conditions such that air or other deleterious gas is prevented from entering. The cooling step may suitably be carried out in a zone in an atmosphere which is generated within the zone by the heat of the container and which is 'mert to or does not deleteriously affect the contents of the container.

One example of a suitable container or box for carrying out the diffusion process is diagrammatically illustrated in vertical section in FIGURE 1 of the accompanying drawings, and one example of a suitable apparatus in which the container can be cooled upon withdrawal from the furnace, is diagrammatically illustrated in FIGURE 2.

The container or box shown in FIGURE 1 comprises a casing l of rectangular section provided on its inner surface with an integral member 2 of angle section which defines a channel 3 extending around the inner surface of the container at its upper end. A lid 4 rests on the upper outer end of the member 2, the lid having an endless depending flange 5 which extends into the channel 3. All parts of the container and lid are of mild steel.

In use, the box is filled with the sand/ chromium composition a in which the strips of mild steel or wrought iron are buried. An appropriate sealing composition, for example, a solution of parts by weight of sodium tetrasilicate (Na Si,,O and 100 parts by Weight of sodium dichromate (N21 D 221 0) in 75 parts by weight of water, is poured into the channel 3 and the lid 4 is placed position with its depending flange 5 extending into the sealing composition 8 to seal the container.

The sealed container is then placed in a furnace, is

3 rought to the appropriate temperature, for example, a temperature in the range l030l200 C., and held at that temperature for a time, for example, 4 to hours, sutiicient to permit the requisite amount of iron in the strips 7 to be replaced by chromium.

Upon completion of the difiusion, the container is withdrawn from the furnace and is allowed to cool in the apparatus shown in FIGURE 2.

The container 1 is placed on supports 10 which provide a free space between the base of the container and a floor or platform 11 which is preferably mounted on wheels (not shown). Walls 13 and 14 form an endless channel 12 on the floor ll surrounding the container 1, the inner wall 13 being of greater height than the outer wall 14. The wall 14 is provided with one or more inlets 15 through which a liquid sealing medium, for example, water, is fed into the channel 12, and one or more outlets in? through which the liquid medium flows out of the channel and which serve to maintain the liquid at a constant level in the channel.

The lower ends of the depending walls of a mild steel canopy or hood 17 extend into the channel 12. The canopy 17 is supported by a series of clips or projections 33 which bear on the inner wall 13, the lower edges of the walls of the canopy being below the level of the outlets 16 but above the base of the channel 12. Alternatively the canopy 17 may rest on the base of the channel 12 provided that gaps or holes are provided in the depending walls of the canopy below the level of the outlets 16 and preferably adjacent to the lower edges of the depending walls.

One or more cups or galleries 19 are provided on the inner surface of the canopy 17 at positions above the clips 13, to hold a composition or compound, for example, ammonium chloride or ammonium carbonate, which upon being heated by the heat released as the container 1 cools, will evolve a gas to provide an atmosphere within the canopy 17 which will not deleteriously affect the diffused articles in the container 1.

In use, a continuous flow of water is passed into and through the channel 12. Upon removing the container '1 from the furnace, it is placed on the supports 10 and the canopy 17 containing an adequate supply of a suitable compound in its cups 19, is lowered into the position shown in FIGURE 2. The compound in the cups 19 decomposes with the evolution of the inert gas or gases which sweep out most if not all of the air from within the canopy, the air bubbling out through the water seal in the channel 12. It the seal of the container iris not adequate to prevent the ingress of gas during the cooling period, then the gas which penetrates into the container 1 will consist substantially wholly of the gas evolved within the canopy and the diffused articles in the container 1 will not be damaged through oxidation.

The cups or galleries 19 are preferably provided adjacent to the top of the canopy 17 and adiacent to the mouth of the container 1 so that even when all of the air is not purged from Within the canopy by the inert gas, the

, mouth of the container is nevertheless surrounded by an atmosphere of the inert gas.

The malleable chromium has many uses. For example, sheets or strips of the malleable chromium may be used for lining reactors, economisers and hue stacks and for forming valve diaphragms.

The invention is illustrated in the following examples.

Example. 1

The sand contained more than 99.3% silicon dioxide and less than 0.05% ferric oxide. Not less than 70% of the sand was of a particle size which passed 72 13.3.5. whilst the sand contained no particles which did not pass The sand/chromium composition was placed in a container of the hind illustrated in FEGURE 1 of the accompanying drawings, and three pieces of suitable ENZA mild steel of 0.003", 0.005 and 0.008" thickness respectively, were buried in the composition. The phosphcus, sulphur, manganese, silicon and carbon contents or" each of the three str'i s were-P 0.03%, S 0.03%, Mn 0.07%, Si 0.01% and C 0.06%. The container was closed and it was sealed with a liquid of the composition given by way of example the description of FIGURE 1. The composition of the liquid seal was such that it was substantially saturated with chromium at the temperature used during diffusion. I

The sealed container was placed in a furnace and its temperature was raised to 1050 (3., the container being held at that temperature for 10 hours. The container was then cooled in such manner as to prevent the entry of oxygen and other deleterious gases into the apparatus, the cooling being carried out in the apparatus illustrated in FIGURE 2 of the accompanying drawings. The compound placed in the cups or galleries 19 for generation of the inert gas, was ammonium carbonate I g After cooling, the three pieces of metal were analysed: Each of the strips was malleable and the chromium con- 'tent of each was found to be as follows:

0.003" thick strip 96% chromium; 0.005" thick strip a 94% chromium; 0.008" thick strip 90% chromium;

Chemically, the behaviourof the three pieces was substantially the same as that of pure chromium;

Example 2 The composition employed consisted of the following ingredients:

% by weight of chromium metal of panticle size below 74.55% by weight of sand; 7

0.15% by weight of ammonium chloride;

0.25% by weight of ammonium fluoride, an

0.05% by weight of ammonium nitrate.

The sand contained more than 99.3% silicon dioxide and not less than 70% of the sand was of a particle size which passed 72 whilst the sand contained no particles which did not pass 32 B.S.S.

The sand/chromium composition was placed in the container illustrated in FIGURE 1, and two pieces of pure wrought iron of 0.008" and 0.005" thickness respectively, were buried in the composition. The container was closed and it was sealed with a liquid of the same 0.005" thick strip 71% chromium.

0.008" thick strip 63% chromium.

Example 3 The composition employed consisted of the following ingredients:

% by weight of chromium metal. of particle size below B.S.S.; V V 64.50% by weight of sand;

0.10% by weight of ammonium chloride; 0.35% by weight of ammonium fluoride, and 0.05 by weight of ammonium nitrate.

The sand contained more than 99.3% silicon dioxide and not less than 70% of the sand was of a particle size which passed 72 B.S.S. whilst the sand contained no particles which did not pass 32 B.S.S.

The sand/chromium composition was placed in the container illustrated in FIGURE 1, and two pieces of suitable ENZA mild steel of 0.003 and 0.005 thickness respectively, were buried in the composition. The phosphorus, sulphur, manganese, silicon and carbon contents of each of the two strips were-P 0.05%, S 0.05 Mn 0.27%, Si 0.08% and C 0.10%. The container was closed and it was sealed with a liquid of the same composition as that used in Example 1.

The sealed container was placed in a furnace and its temperature raised to 1040 C., the container being held at that temperature for 6 hours. The container was then cooled in the manner described in Example 1.

After cooling, the two pieces of metal were analysed. The strips were malleable and the chromium content was found to be as follows:

0.003" thick strip 34% chromium. 0.005" thick strip 31% chromium.

Example 4 The composition employed consisted of the following ingredients:

35% by Weight of chromium metal of particle size below 60 B.S.S.;

64.50% by weight of sand;

0.10% by weight of ammonium chloride;

0.35% by weight of ammonium fluoride, and

0.05 by weight of ammonium nitrate.

The sand contained more than 99.3% silicon dioxide and not less than 70% of the sand was of a particle size which passed 72 3.5.3. Whilst the sand contained no particles which did not pass 32 B.S.S.

The sand/ chromium composition was placed in the container illustrated in FIGURE 1, and two pieces of suitable ENZA mild steel of 0.003 and 0.005" thickness respectively, were buried in the composition. The phosphorus, sulfur, manganese, silicon and carbon contents of each of the two strips were-P 0.05%, S 0.05 Mn 0.14%, Si 0.04% and C 0.09%. The container was closed and it was sealed with a liquid of the same composition as that used in Example 1.

The sealed container was placed in a furnace and its temperature raised to 1120 C., the container being held at that temperature for 5 hours. The container was then cooled in the same manner as that described in Example 1.

After cooling, the two pieces of metal were analysed. The strips were malleable and the chromium content was found to be as follows:

0.003 thick strip 89% chromium. 0.005 thick strip 83% chromium.

The strips or sheets of malleable chmornium produced in the examples can be united to form a larger sheet, for example, by fusion welding or a number of sheets or strips can be formed into a thicker unit or block by cold forging.

The chromium-iron and iron-chromium alloys produced in Examples 1, 2, 3 and 4 exhibit some quite unusual characteristics. For example, upon raising the alloy to 900 C., and then cooling to room temperature, some increase in the ultimate tensile strength of the maerial results. Prolonged exposure of the alloys to 900 C., and determination of the ultimate tensile strength at temperatures above 600 C., shows that although there is a considerable fall in the ultimate tensile strength, pro longed exposure at 900 C. does not produce any further fall in the ultimate tensile strength. This can be contrasted with the recrystallization that takes place in the nickel-chromium alloys after prolonged exposure at these temperatures.

The alloys of the invention do not exhibit the conventional fatigue failure and are more resistant to such failure. After repeated 180 reversals of strips of the alloys, they do not crack and fail except after hundreds of reversals. Thus the strip of Example 1 (the 0.005" thick strip) containing 94% chromium did not crack until it had undergone 310 reversals. The two strips of Example 4, containing 89% and 83% chromium respectively, were each given 200 reversals at which stage neither showed any sign of a crack.

Discoloration of the alloys does not occur until a temperature is reached which is about 180 degrees C. higher than that at which discoloration occurs with conventional l88 chromium-nickel alloys.

I claim:

1. A process for the manufacture of a malleable ironchromium alloy, which comprises subjecting a sheet of material having a thickness not greater than about 0.008 inch and selected from the group consisting of mild steel and wrought iron to chromium diffusion, the temperature and duration of diffusion and the thickness of the sheet and the nature of the said material being such that not less than 20% by weight of the iron in the sheet is replaced by chromium said sheet of material containing, in percentage by weight, not more than about 0.05 0.05 0.5%, 0.15%, and 0.12% of phosphorus, sulphur, manganese, silicon, and carbon, respectively.

2. A process for the manufacture of a malleable ironchromium alloy, which comprises maintaining a sheet of material having a thickness not greater than about 0.008 inch and selected from the group consisting of mild steel and wrought iron in a mixture of finely divided chromium and a sand of high purity at a temperature within the range 10001400 C., the thickness of the sheet and the nature of the said material and the duration of the treatment being such that not less than 50% by weight of the iron in the sheet is replaced by chromium said sheet of material containing, in percentage by weight, not more than about 0.05%, 0.05%, 0.5%, 0.15%, and 0.12% of phosphorus, sulfur, manganese, silicon, and carbon, respectively.

3. A process according to claim 2, in which the content of substances other than iron in the asid material is such that at least 90% by weight of the iron in the sheet can be replaced by chromium during the treatment.

4. A process according to claim 2, in which the thickness of the said sheet and the content of phosphorus, sulphur, manganese, silicon and carbon in the said material are such that not less than 90% of the iron can be replaced by chromium.

5. A process according to claim 2, in which the said sheet is maintained within the said temperature range in the presence of at least one of the members of the group consisting of hydrogen chloride, hydrogen fluoride and nitrous oxide.

6. A process according to claim 2, in which the mixture of chromium and sand contains ammonium chloride, ammonium fluoride and ammonium nitrate in amounts by weight of not less than 0.05%, 0.2% and 0.0 1% respectively of the weight of the mixture of chromium and sand.

7. A process according to claim 2, including the step of cooling the sheet in the mixture of chromium and sand, the cooling being effected in an inert atmosphere which is generated by heat released by the said mixture.

8. A process for the manufacture of a malleable ironchromium alloy, which comprises packing a sheet of material selected from the group consisting of mild steel and wrought iron in a mixture of finely divided chromium and a sand which contains not less than 99.3% silicon dioxide and not more than 0.05% iron estimated as ferric oxide, the thickness of the sheet being not more than about 0.008 inch and the content of constituents other than iron in the said material being such as not substantially to hinder the diffusion of chromium into the sheet at a temperature of not less than 1020 C., heating the sheet and mixture of chromium and sand to a temperature Within the range 10201400 C., and maintaining the sheet and the said mixture Within said temperature range for a period such that not less than 90% by weight of the iron in the sheet is replaced by chromium said sheet of material containing, in percentage by Weight, not more than about 0.05%, 0.05%, 0.5%, 0.15%, and 0.12% of phosphorus, sulfur, manganese, silicon, and carbon, respectively.

9. A process according to claim 8, in which the said mixture of chromium and sand is maintained in said temperature range in the presence of hydrogen chloride, hydrogen fluoride and nitrous oxide.

10. A process according to claim 8, in Which said mixture of chromium and sand contains ammonium chloride, ammonium fluoride and ammonium nitrate in amounts 8 constituting not less than about 0.1%, 0.3% and 0.04% by Weight respectively of the Weight of the said mixture.

References Cited in the file of this patent UNITED STATES PATENTS 1,057,754 Marsh Apr. .1, 1913 1,846,140 Palmer Feb. 23, 1932 1,853,369 Marshall Apr. 12, 1932 1,955,791 Comstock Apr. 24, 1934 2,056,766 Becket Got. 6-, 1936 2,399,848 Becker et al. May 7, 1946 FOREIGN PATENTS 575,676 Great Britain Feb. 28, 1946 7 OTHER REFERENCES Ductile Chromium and its Alloys, Proceedings of a Conference Held at the 1955 Metal Congress, published by the American Society for Metals. (Copy in Div. 3 Patent Ofiice.) Pages 273 and 307 relied upon, 

8. A PROCESS FOR THE MANUFACTURE OF A MALLEABLE IRONCHROMIUM ALLOY, WHICH COMPRISES PACKING A SHEET OF MATERIAL SELECTED FROM THE GROUP CONSISTING OF MILD STEEL AND WROUGHT IRON IN A MIXTURE OF FINELY DIVIDED CHROMIUM AND A SAND WHICH CONTAINS NOT LESS THAN 99.3% SILICON DIOXIDE AND NOT MORE THAN 0.05% IRON ESTIMATED AS FERRIC OXIDE, THE THICKNESS OF THE SHEET BEING NOT MORE THAN ABOUT 0.008 INCH AND THE CONTENT OF CONSTITUENTS OTHER THAN IRON IN THE SAND MATERIAL BEING SUCH AS NOT SUBSTANTIALLY TO HINDER THE DIFFUSION OF CHROMIUM INTO THE SHEET AT A TEMPERATURE OF NOT LESS THAN 1020*C., HEATING THE SHEET AND MIXTURE OF CHROMIUM AND SAND TO A TEMPERA- 